For the HSC you don't need to know how to program in any language. As long as you understand the programming structures (see the course specs and are able to write pseudocode (English-like statements with defined rules of structure and keywords) or are able to draw a flowchart (diagrammatic method representing algorithms) you should be okay for the HSC.
However I've found it helpful to know how to program in a language similar to pseudocode, such as Pascal & Python.
The best way to prepare for the HSC exam is by practising past papers in exam conditions.
Gigacube's right, but if your teacher opts to do a major, you're gonna need to learn a language. This may be taught to you by your teacher, or you can choose to learn one yourself. It just has to be an approved language.
I personally suggest lua. I found it really easy to learn the basics of, and there are ways to emulate more complex structures like objects and classes if you so wish.
Learning a language definitely helped me with my algorithms, so it's worth the effort .
SDD comes no where near actual software development and even programming from the looks of it, and as said above, just understand how to write pseudo code and everything Gigacube said.
As far as real world programming goes though, if you guys are wanting to get into the industry then I'd suggest getting into languages like C++/C#/Java as soon as possible (as well as HTML/CSS/Javascript/SQL), but dependant on where you want to go with it.